<!doctype html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport"  content="width=device-width">
    <title>Document</title>
</head>
<style>
    a{
        text-decoration: none;
        color: pink;
    }
    *{
        margin:0px;
        padding:0px;
    }
    .all{
        width:960px;
        margin:0 auto;
    }
    .top,.bot{
        height:100px;
        background:#ccc;
    }
    .s{
        height:400px;
    }
    .s1{
        background:red;
        width:260px;
        float:left;
    }
    .s2{
        background:green;
        width:500px;
        float:left;
    }
    .s3{
        background:blue;
        width:200px;
        float:left;
    }
    .bot{
        clear:both;
    }
</style>
<body>
<div class="all">
    <a href="index.html">作业一</a>
    <a href="demo.html">作业二</a>
    <a href="home.html">作业三</a>
    <div class="top"></div>
    <div class="cnt">
        <div class="s s1"></div>
        <div class="s s2"></div>
        <div class="s s3"></div>
    </div>
    <div class="bot"></div>
</div>
</body>
</html>
